In optical systems of prior art, the structure of one useful form of semiconductor lasers is a vertical cavity (or simply "vertical") laser. In a vertical laser, there is an active region in a semiconductor body (substrate) which includes a planar pn junction. Typically the plane of this pn junction is parallel to a major surface of a semiconductor substrate body, the major surface of the substrate being considered arbitrarily to be horizontal. In a vertical laser, light is emitted from the top or the bottom (major) surface, or both, of the semiconductor body, a vertical optical cavity being created therein by virtue of reflecting optical mirror(s) located on the top or bottom surface thereof, or both.
The structure of a vertical laser can be made circularly symmetric. Therefore, a vertical laser can have the advantage of relatively low astigmatism. Also, because a vertical laser can be made with a relatively large aperture, it can have the further advantage of a low divergence of the emerging beam as compared with other laser, such as "edge" lasers in which light is emitted from a side edge of the semiconductor body.
A vertical laser typically is built as a double heterostructure (two junctions between chemically dissimilar materials), for example, by successive epitaxial growth of the following semiconductor layers in spatial sequence upon a semiconductor substrate: the bottom mirror, a bottom optical cladding region, the active region, a top cladding region, and the top mirror. Typically, in a vertical laser each mirror(s) is formed by a quarter-wavelength stack, such as a mirror stack formed by alternating layers of two semiconductor materials with differing chemical compositions and hence differing refractive indices, which thus form a semiconductor superlattice. The choice of the semiconductor materials for the mirror stack is made so as to result in large differences in these refractive indices, in order to maximize optical reflectivity and hence minimize the number of periods in the superlattice, and thus minimize undesirable vertical electrical resistance and unwanted power dissipation.
In an optically pumped semiconductor laser, optical radiation of wavelength(s) shorter than that (those) to be emitted by the laser is directed upon the laser to create an electronic population inversion. In a typical electrically pumped (driven) vertical cavity semiconductor laser, electrical current is passed between a top electrode formed on the top major surface of the top mirror and a bottom electrode formed on the bottom major surface of the semiconductor substrate. Many such vertical lasers can be built on a single such substrate, as by trench or other isolation, in such a way that the intensity of light--e.g., ON vs. OFF--emitted by each laser can be controlled by an electrical signal independently of all other lasers on the substrate. Thus, vertical lasers appear especially attractive for use in practical applications where more than one independently controllable source of light is desired on a single substrate. Alternatively, many separate lasers can be mass produced from the single substrate, as by masking and etching apart the individual lasers.
In prior art, the semiconductor substrates that have been used for double heterostructure vertical lasers have been mostly gallium arsenide or indium phosphide. It has been believed to be necessary to build such lasers with very nearly lattice matching of the double heterostructure, including the mirror(s), in order to achieve the high quality (low defect density) epitaxial growth needed for the desirably low optical absorption and high quantum efficiency of light emission. Consequently, the choice of materials for the mirror stack has been limited, in order at the same time to preserve the large difference in refractive index between contiguous layers in the stack. In turn, this limited choice of semiconductor materials usually results in undesirably large conduction and valence band edge discontinuities, whereby undesirably high electrical resistance is exhibited by the mirror stack, owing to the resulting high quantum reflection coefficients and other charge transport barriers ("effective barriers") for both electrons and holes at the interfaces of contiguous layers in the mirror stack. In turn, this high resistance results in undesirably high power loss (dissipation) in the laser.
It would therefore be desirable to have a vertical laser which mitigates the problem of high electrical resistance and power dissipation.